Răspuns:
abc7 - nr initial
abc - nr format dupa stergerea cifrei 7
cifrele sunt in baza 10
abc7-abc=1816
1000a+100b+10c+7-(100a+10b+c)=1816
1000a-100a+100b-10b+10c-c = 1816-7
900a+90b+9c=1809
9(100a+10b+c)=1809
100a+10b+c=1809:9
100a+10b+c=201
1816+201=2017 nr initial